Contact switches are widely used in domestic appliances, such as cooking appliances. The contact switches are disposed beneath a cover on the appliance and a user acts on them by exerting pressure on or pressing the cover, the pressing action being detected thanks to the contact switches.
Different types of contact switches, such as capacitive or optical switches, are known. In optical switches an optical sensor is disposed beneath the cover, the optical sensor generally comprising a radiation emitter that emits radiation (light) and a receiver that may receive at least part of the radiation emitted by the emitter. When a user exerts pressure on the cover that covers the switch, the radiation emitted by the emitter that the receiver receives varies, the pressing action being detected.
In conventional optical switches it is not possible to detect if the optical sensor is working correctly or not, which could cause serious safety problems. If the switch is activated and this is not detected by the optical sensor, this is extremely serious, the result being that the deactivation of the switch is not detected when the user presses it again. For example, United States Patent Application published as US2006/0282070A1 discloses the use of an evaluation circuit for evaluating the functioning of the optical sensor and thereby identify any faults in it, the circuit enabling the evaluation of the value of an input signal in a microprocessor for a preset period of time. The period is divided into a first and a second time interval, the values of the input signal being obtained in one or the other interval. Thus, in the first interval the value mainly depends on the radiation emitted by a light emitter whereas in the second interval it mainly depends on a signal emitted by the microprocessor itself.